The invention relates to a method of providing an electric current taker made from silver or silver-based alloy and having a highly electroconductive contact surface onto a surface of a first end of an aluminium support bar to be used in electrolysis, which support bar via the electric current taker is to be supported against a busbar used in connection with an electrolysis basin.
The invention also relates to an aluminium support bar to be used in electrolysis, whose first end comprises an electric current taker made from silver or silver alloy and having a highly electroconductive contact surface, which support bar via the electric current taker is to be supported against a busbar used in connection with an electrolysis basin.
From FI patent publication 114926 a method is known wherein an electric current taker made from silver or silver alloy is provided at an end of a support bar by coating, using thermal spray coating. The electric current taker forms a metallurgic bond with aluminium. The thermal spray coating requires a highly skilled performer. Further, the end result is not particularly good in terms of electroconductivity and strength since in practice it is very difficult to achieve a good metallurgic bond between aluminium and silver with good electroconductivity. Prior to coating, the materials to be coated have to be cleaned from oxide layers e.g. by sandblasting or wire brushing since otherwise no good contact/joint can be obtained with the coating. The cleaning work is time-consuming and does not always ensure a good end result. After the coating, a short thermal treatment may be carried out to strengthen the joint. Naturally, the thermal treatment adds to the amount of work. FI patent publication 114926 also discloses that the aluminium bar may first be coated with copper, after which a final coating is carried out with silver or silver alloy. The latter procedure is complicated.
From FI patent publication 114927 a method is known wherein an electric current taker made from silver or silver alloy is provided at an end of a support bar by coating, whereby the coating is made on top of a copper contact piece adhered to the end of the support bar, and using a transmission layer which forms a metallurgic joint with the copper contact piece. The transmission layer is made from tin or a tin-based alloy and it is brought onto the contact piece by soldering. The electric current taker is provided on top of the transmission layer either by soldering or by thermal spray coating. The process of adhering the copper contact piece to the aluminium bar increases the manufacturing costs of the support bar. Providing the electric current taker by thermal spray coating requires a highly skilled performer.
From WO publication 2006/117425 a method is known of attaching a silver piece to an end of an aluminium support bar so as to achieve an electric current taker. The purpose of the method is to produce an eutectic reaction between the silver piece and aluminium. It is difficult to attach the silver piece to the underlying aluminium surface. The aluminium bar has to be heated in stages and after heating an oxide layer has to be removed therefrom before the silver piece is attached thereto. The removal of the oxide layer, i.e. cleaning, is carried out e.g. by abrasion. In order to control the oxidation reactions, it is advisable to carry out the heating in a reductive atmosphere, the creation of which naturally involves special arrangements as compared to being able to carry out the heating in the surrounding air. There is not much tolerance in the heating temperatures of the support bar, either, which is why the heating requires a highly accurate performer. In addition, the silver piece has to be pressed at a certain pressure against the heated aluminium surface in order to produce a joint. Advisably, the pressing is carried out in a spot-like manner and repeated cyclically. All in all, the work requires a highly accurate performer and takes a lot of time.